Pulses of prolactin (PRL) and a metabolite of prostaglandin F2α (PGFM) were determined from hourly blood samples collected before, during, and after luteolysis (. n=. 7 heifers). Progesterone concentrations were used to partition the results into six 12-h sets from 12. h before to 36. h after luteolysis. Pulses of PRL with a nadir-to-nadir interval of 4.4. ±. 0.2. h were detected in each 12-h set. Pulses were rhythmic (. P